The present invention relates to the field of sheet handling cylinders for printing presses and more particularly, to such cylinders as are used to transfer sheets between printing stations or to deliver freshly printed sheets for stacking.
In rotary offset printing, freshly printed sheets are transported from one printing station to the next and finally, delivered to a sheet stacker. The transport and delivery steps are accomplished by sheet transfer devices, usually in the form of special purpose cylinders, which include an arcuate gap to accommodate the paper feed grippers. The freshly printed sheets are subject to ink tracking and marking if not handled with great care. Traditionally, this problem is mitigated by minimizing contact pressure and/or active contact area with the transfer cylinders. Sheet support wheels in the form of thin disks with serrated edges, to minimize contact area, are frequently provided as xe2x80x9cskeleton wheelsxe2x80x9d. Even so, ink tracking and marking continue to be a persistent problem. An opposite approach to minimizing the contact area by using multiple skeleton wheels is the use of a full width, low friction cylinder for reduced contact pressure and sheet drag. This has proven to be more effective for the intended purpose, particularly when the cylinder is covered with a soft fabric jacket to provide a cushioning effect.
U.S. Pat. No. 4,402,267 discloses the use of low friction coating on the surface of a transfer cylinder, over which a covering or jacket of flexible material is loosely fitted. It is disclosed that the low friction coating of the transfer cylinder permits relative movement to take place between the jacket and cylinder surface, and that this freedom of relative movement contributes significantly to tracking/marking prevention.
U.S. Pat. No. 6,073,556 discloses that relative movement between the jacket and cylinder surface is impeded by the build-up of a static electric charge. The low friction transfer cylinder coating of the prior art acts as an insulator, so that friction induced static electricity from the printed sheets accumulates on the surface of the transfer cylinder. As a consequence of this electrostatic charge transfer and accumulation, the flexible jacket covering tends to cling to the underlying cylinder surface and lose its freedom of movement. 6,073,556 teaches the inclusion of a conductive agent such as carbon black, or the like, in the low friction coating of the transfer cylinder for dissipation of the above referenced static charge. Thus, relative movement between the jacket and cylinder surface is unimpaired.
A commercially available system sold under the xe2x80x9cNO-MARKxe2x80x9d name employs a semi-rigid plastic sheet blanket having a proprietary, micro-beaded silicone surface and a full surface backing of loop VELCRO. xe2x80x9cSkeleton wheelsxe2x80x9d, in the form of thin, segmented discs, with peripherally applied strips of hook VELCRO, are spaced along a transfer cylinder shaft. The blanket is wrapped around the skeleton wheels and held in place by the connection of VELCRO loops and hooks. Additional support is provided to the blanket by a series of longitudinal support members, set flush with the skeleton wheel edges. An ink repellent jacket is not required for this system.
Conventional rotary printing cylinders are machined from somewhat complex aluminum castings. The basic cost of the cast and precision machined cylinder, plus the cost of the conductive, low friction coating and the jacket is significant so that a typical, non-marking cylinder assembly will sell for upwards of $800.00. This is partially because variations in width, shaft size and cylinder diameter are such that virtually every cylinder is a custom design for a specific printing press make and model, mandating small volume production runs and high costs at the factory. This also creates inventory and availability problems for replacement parts at the dealers and repair shops.
Therefore, a first object of the present inventions is to provide more economical, non marking transfer cylinder apparatus, in a form that lends itself to the use of relatively inexpensive and readily available materials. A second object of the present inventions is to provide transfer cylinders in standardized configurations, so that only a few sizes can be retrofitted to a broad range of printing machines. A third object of the present inventions is to provide serviceable, non-marking transfer cylinders, that require little maintenance and are capable of surviving paper jams and abuse without sustaining permanent damage. A fourth object is to provide non-marking transfer cylinders that are easy to assemble and install. A fifth object is to provide a cylindrical jacket with strong tension.
The present inventions contemplate apparatus for improved non-marking transfer cylinders as used on rotary printing presses. These inventions relate to or employ some steps and apparatus well known in the rotary printing press arts, and thus not the subject of detailed discussion herein.
Rotary printing press transfer cylinders of the present inventions have one or more axially spaced apart discs, with a radiused major segment and an open minor segment having inwardly disposed edges that join into a central opening. Each disc has a shaft receiving hub at the central opening, mounted concentric with the major segment diameter and sized to receive a central, rotating shaft. A flexible, plastic sheet or thin metal covers the major disc segments, so as to provide a cylindrical shape, and this sheet has axially oriented flanges, bent inwardly to fit within the inwardly disposed minor segment edges. Attaching members, preferably including a flexible central portion, hook into holes in the cover sheet flange, to pull the sheet closely against the outside diameter of the major segments. The attaching members are tensioned and held in place with respect to the spaced apart discs by a removable mounting member or by attachment to the central shaft. An ink repellent jacket, attached over the cover sheet, provides cushioned, non-marking contact with printed sheets for transfer in a manner well known to those skilled in the printing arts. While many variations in the manner of attachment of the cover sheet flanges to the discs and the jacket to the cover sheet are possible, only preferred apparatus and methods are disclosed.
The cover sheet may be jacketed with any available anti-making material. A preferred method is an ink repellent, cushioning blanket of fabric or webbing such as is commercially available from PRINTING RESEARCH, INC., in Dallas, Tex. or from BBA NONWOVENS in Simpsonville, S.C. There are some in the art who espouse loose attachment of the blanket to the cylinder, so as to permit relatively free movement of the blanket with respect to the cylinder surface for better non-marking performance. However, it has been determined that, at least in the case of the present inventions, such freedom is not necessary to non-marking performance. Thus, such blankets can be held in place around the cover sheet of the present inventions in a variety of ways, including the use of VELCRO, double-sided tape, a spray-on adhesive or mechanical fasteners. In the preferred embodiment, adhesive backed VELCRO strips applied to the cover sheet flanges and inside of the extended ends of the cover sheet provide hooks that engage the mesh of the jacket material so as to hold it in place. As an alternative, that portion of the blanket which extends beyond the cylinder ends may be secured by spring clips inserted within the extended cylinder ends so as to bear against the folded-under blanket edges. Another option is the use of double-sided adhesive strips applied to hold the folded-under blanket edge in place.
Yet another aspect of the present inventions is the clearance dimension between the transfer cylinder and the impression cylinder from which printed sheets are taken. It has been determined empirically that, at least for the present inventions, as the weight of the printed paper stock increases, so does the best clearance for non-marking. The preferred clearance is found to be in the range of 0.38xe2x80x3 for 20 lb. paper and increases up to 0.75xe2x80x3 for 100 lb. cover stock. The xe2x80x9c0.25xe2x80x9d industry standardxe2x80x9d clearance dimension is workable for 20 lb. paper stock, but notably less effective for the intended purpose of non-marking when transferring heavier stocks.